A magnetic polyester tape is usually prepared by applying a magnetic coating on a biaxially stretched polyethylene terephthalate film by means of a calender roll and then slitting the film into a plurality of narrow tapes. It is the most serious problem in the step of applying the magnetic coating that the so-called "debris" is generated. The debris is polyethylene terephthalate swarf which results from the abrasion of polyethylene by calender rolls when the film is passed through the rolls. This fact is described in detail in Japanese Patent Publication No. 43-23888, which is incorporated herein by reference.
Another important problem is protrusive deformation. When a film under tension is wound up, foreign matters happen to be included between adjoining turns of the film so that subsequent overlapped turns of the film may be locally protruded. The film is deformed in a loop-like shape at locally protruded portions. Such deformation prevents the uniform application of a magnetic coating, which may result in the occurrence of drop out upon use as a magnetic tape.
The friction coefficient of a film is still another problem to be considered. Since the running performance of a magnetic tape set on a tape recorder depends upon the friction coefficient, the latter should be maintaned as low as possible.
While various methods for improving all the above three properties are known, most effective is to introduce fine particles in a film to roughen the film surface. In general, fine particles can be added according to the following two methods.
In the first, precipitation methods, a catalyst such as a calcium or a lithium compound which is used in an ester-interchange reaction is precipitated during the polycondensation reaction in the form of fine particles of, in a case where calcium is used, calcium terephthalate and an oligomer calcium salt. In the second, addition method, inert fine particles are added to the reaction system.
The first method has advantages in that equipment for classifying and dispersing particles are unnecessary and the filter life (the life of a filter placed in an extruder) can be maintained for a longer period of time. However, it is difficult to control the amount and particle size of particles precipitated and particles included are inclined to be broken during the film forming process so that stable properties may not be expected in the resulting polymer film. On the contrary, the second method has advantages in that particles added are hardly broken and the particle size can be controlled so that stable properties may be expected, while it is disadvantageous in that the filter life is inclined to be reduced. Even when a film is provided with improved properties, it cannot be used in practice unless the filter life is long enough.